EX1-3 - Thermal Remote Sensing

Question 1

Categories within the IR region

Answer 1

Near IR: .72 microns - 1.30 microns
Mid IR: 3.5 microns - 4.5 microns
Far IR: 3 microns - 14 microns

 Far IR; Thermal IR; Emissive IR  all refer to the same
portion of the EM spectrum

Question 2

Optical vs thermal RS

Answer 2

Optical RS (visible, NIR) senses the differences in the abilities of different objects to
reflect solar radiation
Thermal RS (Far IR) senses the differences in the abilities of different objects to emit
thermal radiation
All objects emit thermal radiation

Question 3

How is thermal remote sensing different from optical remote sensing

Answer 3

 Instruments / imaging devices used are different
 Interaction of these wavelengths with atmosphere is different
 No scattering (as in shorter wavelengths)
 Kind of information acquired by sensing the thermal IR is
different
 Thermal IR captures variations in emitted energy, thereby
providing information regarding:
 Surface temperatures
 Thermal properties of surface materials (soils, rocks, vegetation, manmade
structures). This helps identify surface materials

Question 4

Thermal windows

Answer 4

Visible .4-.7 microns
Reflective IR: (.07 - 1.3)NIR & (1.3-3) MIR
Thermal IR: 3-5 and then 8-14

Question 5

Sensitivity of thermal radiometers (also, why spatial resolution of thermal bands are poorer)

Answer 5

• Thermal radiometers measure intensity of
thermal radiation
• Spatial resolution of a radiometers if
determined by the IFOV
• Since less energy is available in the IR, the
IFOV is generally larger than VIS, NIR
bands. Without large IFOV (60 m – 1000
m), insufficient radiance would be
available.
• Larger IFOV -> lower spatial resolution ->
more energy reaches the detector -> high
S/N ratio -> better measure of radiance ->
high radiometric resolution

Question 6

Thermal properties of objects – emissivity, kinetic temperature and radiant temperature and the relationship between these

Answer 6

 Heat is the internal energy of a substance arising
from its atomic and molecular motion.
 Unlike heat, temperature is a measure and NOT a
form of energy
 Kinetic Temperature / True temperature measures
the average kinetic energy (atomic and molecular
motion) in the body, and is measured using a
thermometer.
 Radiant Temperature / Apparent Temperature
measures the emitted energy of an object

 Radiant temperature measures the energy emitted by an object
 This signature is a function of emissivity and kinetic temperature
 It is the kinetic temperature reduced by some amount due to the emissivity of the object
 Thermal RS measures radiant temperature
 Hence, knowledge of emissivities is very important to reach conclusions regarding the
kinetic temperature of objects
 Problem: Oftentimes, emissivities are not known

Question 7

Thermal inertia and its implications for thermal RS

Answer 7

 Thermal inertia measures the tendency of a substance to resist changes in
temperature. It is the resistance of an object to changes in the thermal/radiative
environment

 Determinants include: mineral characteristics, lithification, presence of unconsolidated materials (sand, dust, loose sediments)

Objects that change T greatly over the
day have low thermal inertia
(e.g., sand, concrete and rocks) and
generally stand out during the night
hours relative to objects with high
thermal inertia (water)

P = (K * p * c)1/2

P = thermal inertia, 
K = thermal conductivity
p = is density, c = thermal / heat capacity

Question 8

Diurnal and seasonal temperature variations for different objects and its implications for thermal RS

Answer 8

shows that FAR IR images interpretation change thermally by season.

In the northern hemisphere
March: Land cool (darker) relative to water
November: Water cool (Darker) relative to land

Question 9

Some applications of thermal RS

Answer 9

SST
Coral Bleaching
El Nino
Urban Heat Island